One well-known method used in the past for treating the ocular disorder known as a cataract involves removing the lens of the eye, and employing an intraocular lens as a substitute for the lens. An intraocular lens of this kind is used by being inserted into the capsule from which the lens has been removed. The lens has an optical portion, which functions in place of the lens of the eye, and a support portion for positioning and immobilizing the optical portion within the capsule.
In the area of cataract treatment, there have been numerous improvements in relation to intraocular lens design and surgical techniques for the purpose of reducing demands on patient imposed by surgery and for avoiding subsequent complications. However, even now, it cannot be said that the probability of subsequent complications is low. Secondary cataracts are known to occur with high frequency as a subsequent complication of cataract treatment. A secondary cataract is a subsequent complication that arises due to proliferation of epithelial cells within the capsule from which the lens of the eye has been removed, resulting in clouding of the posterior capsule within the optical zone of the intraocular lens.
Various research has been carried out for the purpose of preventing such secondary cataracts, and one outcome of this research has been to elucidate that rates of incidence of secondary cataracts will differ depending inter alia upon the shape of the intraocular lens. Specifically, it has been shown that by maintaining the outer fringe portions of an intraocular lens so that they are pressed against the posterior capsule, infiltration of epithelial cells into the optical zone can be blocked, making it possible to inhibit the incidence of secondary cataracts.
Another recent development has been the proposal of intraocular lenses of one-piece design, in which the optical portion and the supporting portions are integrally constituted using the same material. Such one-piece type intraocular lenses are beginning to attract attention for reasons such as the fact that they are easier to manufacture than conventional intraocular lens of three-piece type in which, after forming pair of supporting portions separate from the optical portion, these supporting portions are affixed to the optical portion.
However, for reasons relating to design, intraocular lens of one-piece type have the problem that, as compared with intraocular lenses of conventional three-piece design, it is more difficult to keep the outer fringe portions of the lens securely pressed along its entire circumference against the posterior capsule during wear. Therefore, an inherent problem is that it will be more difficult to inhibit secondary cataracts, as compared with intraocular lenses of the conventional three-piece design.
Specifically, in the edge contour constituting the line of intersection of the posterior surface and the outside edge surface of the optical portion of an intraocular lens of conventional three-piece design, a distinct continuous edge part is formed along the entire circumference. Thus, by pressing this edge part against the posterior capsule, it is a simple matter to maintain the outer fringe part of the optical portion securely and stably in contact along its entire circumference against the posterior capsule. In an intraocular lens of one-piece type, however, the supporting portions are formed projecting with generally unchanging thickness dimension from the outer fringe part of the optical portion. Thus, in the areas where the supporting portions are formed, no edge contours are present at the outer fringe part of the optical portion. It is therefore exceedingly difficult to keep the optical portion pressed firmly against the posterior capsule in the areas where the supporting portions are formed, and as a result epithelial cells can easily infiltrate through these areas, reducing the effectiveness in inhibiting secondary cataracts.
Intraocular lenses of one-piece type for addressing this problem have been proposed in Patent Document 1 (WO/2004/096099A1) and Patent Document 2 (JP-A 2003-504115).
According to the intraocular lens taught in the former Patent Document 1, the thickness dimension of the supporting portions formed projecting out from the outer circumferential surface of the optical portion is designed to be smaller than the thickness dimension of the outer circumferential surface of the optical portion and is disposed with bias towards the anterior surface of the optical portion, thereby forming an edge-shaped edge contour around the entire circumference of the back edge of the outer circumferential surface of optical portion.
However, in the intraocular lens of the design taught in Patent Document 1, the thickness dimension of the supporting portions will inevitably be smaller than the thickness dimension of the outside edge surface of the optical portion. Thus, it will be difficult to achieve sufficient strength of the supporting portions, and due to insufficient strength in the supporting portions it will be difficult to maintain shape stability and positioning stability during wear; moreover, there will be a risk of insufficient pressing force of the edge contour against the posterior capsule, which is derived from the action of force transmitted through the supporting portions. Furthermore, it will be necessary to increase the thickness dimension of the optical portion in order to ensure strength of the supporting portions, creating the problem of greater imposition on the patient due to the fact that the incision wound made in the capsule during surgery must be made larger, in order to accommodate the thicker optical portion.
The intraocular lens disclosed in the latter Patent Document 2 is designed so that the supporting portions formed projecting from the optical portion are thick at the basal end only, thus forming on the posterior surface of the supporting portion a shoulder portion situated at a location in proximity to the optical portion; this shoulder portion is utilized to constitute an edge contour at locations where the supporting portions are formed. According to the design disclosed in Patent Document 2, by forming the supporting portions so as to project on a slope towards the anterior side, it is possible to avoid making the supporting portions thinner, while at the same time forming a shoulder portion on the posterior surface of the supporting portions in their medial section.
However, since the shoulder portion is formed on the back face of the supporting portions in the medial section in their direction of projection, the shoulder portion will connect the two widthwise edges of the supporting portions at either edge across their width, in a generally orthogonal geometry. Thus the edge contour, which has been formed utilizing the posterior surface of the supporting portions from the outer fringe part of the optical portion, will have a generally right-angled inflection point. When the edge contour is pressed against the posterior capsule, the presence of such an inflection point will tend to produce in the posterior capsule irregular wrinkle-like deformation centered around the inflection point. There is a risk that due to this irregular deformation, the surface of the posterior capsule will fail to follow the shape of the edge contour that has been formed in the outer fringe part of the back faces of the optical portion and the supporting portions, creating a gap or the like. Additionally, since the posterior capsule is not only flexible but also of spherical shape, and since the inflection point is positioned furthest towards the fringe, the inflection point may catch on the posterior capsule and produce a gap between the edge contour and the posterior capsule, at a location lying towards the center from the location pressed against by the inflection point.    Patent Document 1: WO2004/096099A1    Patent Document 2: JP-A 2003-504115